1. Field of the Invention
The present invention relates to a method and system for detecting instability of formations through which a borehole passes and in particular to measuring change in a logable characteristic as an indicator of the change in the tendency of a formation to swell.
2. Description of the Background
There are many factors to be considered in drilling a borehole in the earth. One important factor which must be considered is the stability of the various formations through which the borehole passes. There are well known methods and apparatus for dealing with unstable formations, and it becomes a significant factor in determining which method and apparatus will be the most appropriate and when they must be utilized. For example, one way of dealing with an unstable formation is to change the drilling mud. Some drilling muds will have less of a tendency to invade and/or react with a surrounding formation than another drilling mud, the difference being primarily due to the chemical composition of the drilling mud and its relative density. Most drilling muds contain chemical compounds which keep the mud clays in suspension. These same compounds react with shale formations causing them to break down. This could lead to failure of the borehole if corrective action is not promptly instituted.
The detection and prevention of borehole failure resulting from enlargement (sloughing) or contraction (tight hole) is an important factor in the successful drilling of a well. Borehole instability occurs when a stress in the formation at the borehole wall exceeds the strength of the formation. The interaction of the drilling muds, in particular with shale formations, can result in swelling of the shale and failure of the well bore. The swelling of shales constitute a major cause of borehole instability since most sands and limestones are not affected by normal drilling muds. The measurement of the initial shale properties and subsequent change in these properties would allow the direct monitoring of borehole stability and lead to a substantial reduction in the cost of drilling.
There have been attempts to monitor swellings of formations, due to adsorptions of drilling muds and pressure changes. However, these have all had several major problems including the fact that they depended upon particles returned to the surface and required wet chemistry analysis. It is substantially impossible to determine exactly where the particles originated from in the borehole. Also, there are substantial delays associated with wet chemistry analysis.